U.S. patent number 7,216,510 [Application Number 10/636,717] was granted by the patent office on 2007-05-15 for method for the forming of glass or glass ceramics.
This patent grant is currently assigned to Schott AG. Invention is credited to Thorsten Doehring, Hauke Esemann, Eva Hoelzel, Ralf Jedamzik.
United States Patent |
7,216,510 |
Doehring , et al. |
May 15, 2007 |
Method for the forming of glass or glass ceramics
Abstract
A process for forming glass or glass ceramics is disclosed,
wherein a glass ceramics form (12) is made from a starting glass by
molding, which is transformed by a heat treatment into a keatite
glass ceramic comprising predominantly keatite mixed crystals. With
such a keatite glass ceramics form (12) formed bodies can be
prepared from blank parts by sagging under gravity force at a
temperature above the glass transition temperature of the blank
part (14) (FIG. 1).
Inventors: |
Doehring; Thorsten (Mainz,
DE), Jedamzik; Ralf (Griesheim, DE),
Esemann; Hauke (Woerrstadt, DE), Hoelzel; Eva
(Ober-Olm, DE) |
Assignee: |
Schott AG (Mainz,
DE)
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Family
ID: |
30775521 |
Appl.
No.: |
10/636,717 |
Filed: |
August 7, 2003 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20040107731 A1 |
Jun 10, 2004 |
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Foreign Application Priority Data
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Aug 16, 2002 [DE] |
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102 38 607 |
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Current U.S.
Class: |
65/107; 65/106;
65/273 |
Current CPC
Class: |
C03B
19/02 (20130101); C03B 23/025 (20130101); C03B
23/0252 (20130101); C03B 23/0357 (20130101); C03B
32/02 (20130101); C03B 40/00 (20130101); C03C
10/0027 (20130101); G21K 1/067 (20130101); G21K
2201/064 (20130101); G21K 2201/067 (20130101) |
Current International
Class: |
C03B
23/025 (20060101) |
Field of
Search: |
;65/107 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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199 38 811 |
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Sep 2000 |
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DE |
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0 060 460 |
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Sep 1982 |
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EP |
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1 170 264 |
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Jan 2002 |
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EP |
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Other References
Pfaender, Heinz G., Schott-Glaslexikon, 1997, pp. 26 and 27. cited
by other .
Author Unknown; "Development of Segmented X-ray Mirrors for the
XEUS Mission"; 2001. cited by other .
American Ceramic Society Bulletin; "Li20-Al203-Si02
Glass-Ceramics"; pp. 1926-1930; 1989. cited by other .
M.H. Lewis, J. Metcalf-Johansen, P.S. Bell; "Crystallization
Mechanisms in Glass-Ceramics"; 1977; pp. 278-288. cited by
other.
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Primary Examiner: Hug; Eric
Assistant Examiner: Dehghan; Queenie
Attorney, Agent or Firm: Harness, Dickey & Pierce,
P.L.C.
Claims
The invention claimed is:
1. A method for the hot forming of X-ray mirror substrates from a
borosilicate glass comprising the following steps: preparing a mold
from a lithium-aluminosilicate base glass by casting and subsequent
heat treatment for transforming into a keatite glass ceramic
comprising at least 80 vol.-% of keatite mixed crystals;
mechanically finishing the mold to a desired shape and surface
quality; preparing a blank plate of a borosilicate glass for hot
forming; polishing said blank plate on both surfaces; sagging said
blank plate under gravity force onto said mold at a temperature
above the glass transition temperature of the blank plate; and
cooling the blank plate, after sagging, to room temperature at a
controlled cooling rate of about 0.1 K/min, thereby producing a
formed body.
2. The method of claim 1, wherein sagging of the blank onto the
glass ceramic mold is assisted by applying a vacuum.
3. The method of claim 1, wherein sagging of the blank onto the
glass ceramic mold is assisted by applying an excess pressure.
4. The method of claim 1, wherein producing of the formed body is
performed under clean room conditions.
5. The method of claim 1, wherein the glass ceramic mold is
designed concavely when the coefficient of thermal expansion of the
mold is smaller than that of the blank.
6. The method of claim 1, wherein the glass ceramic mold is
prepared with a Wolter's profile.
7. The method according of claim 1, wherein the formed body after
having been formed is mechanically finished.
8. The method according of claim 1, wherein a reflective coating is
applied to the formed body.
9. The method of claim 1, wherein said blank plate of borosilicate
glass is one of the group consisting of BOROFLOAT.RTM.
(borosilicate glass) glass plate, a DURAN.RTM. (borosilicate glass)
glass plate and a PYREX.RTM. (borosilicate glass) glass plate, and
wherein said sagging step is performed at a temperature between
550.degree. C. and 850.degree. C.
10. A method for the hot forming of glass or glass ceramics
comprising the following steps: preparing a mold from a
lithium-aluminosilicate base glass by casting and subsequent heat
treatment for transforming into a keatite glass ceramic mainly
comprising keatite mixed crystals; providing a blank plate of glass
or glass ceramics for hot forming; polishing said blank plate on
both surfaces; sagging said blank plate under gravity force onto
said mold at a temperature above the transition temperature of the
blank plate; cooling the blank plate, after sagging, to room
temperature at a controlled cooling rate of about 0.1 K/min.
Description
BACKGROUND OF THE INVENTION
The invention relates to a method for the forming of glass or glass
ceramics, in particular for the preparation of mirror substrates,
wherein a formed body is prepared from a blank by sagging under
gravity force onto a mold.
The invention further relates to a method for preparing a mold
suitable therefore.
From DE 199 38 811 A1 a method for the forming of glass ceramic
parts is known according to which a blank is sagged under the
influence of infrared irradiation, while possibly applying a vacuum
or an excess pressure.
Such a forming method is suitable for forming a large number of
blanks in three dimensions. Thus, e.g. trough-like or chamfer-like
parts having a cross-section in circle segment form, rectangular,
trapezoid or other form can be prepared.
The precision of the parts manufactured thereby naturally depends
on the precision of the mold that is utilized. Up to now according
to this method only mass-produced parts have been manufactured,
while utilizing mostly metallic molds.
For the production of telescopes for scientific X-ray satellites up
to now polished solid cylindric mirror shell substrates made of the
glass ceramic ZERODUR.RTM. (glass ceramic, Schott Glas) have been
utilized as projection optics. Due to the low reflectivity of all
known materials for high-frequency X-rays at normal angle of
incidence X-ray mirrors and X-ray telescopes are, preferably,
operated at grazing incidence while taking advantage of the
physical effect of total reflection. To this end two-piece
telescopes for two reflexions are utilized, wherein the cylindric
mirror shells have the specific forms of a parabola and a hyperbola
which, according to Wolters, are particularly suited for such
applications. For the telescopes of the scientific satellites ROSAT
and CHANDRA polished solid cylindric mirror shell substrates of
ZERODUR.RTM. (glass ceramic) were utilized as imaging optics. By
contrast, for the X-ray satellite XMM-NEWTON galvanically produced
nickel shells having a similar Wolters profile were utilized as
mirror substrates. Up to now all X-ray satellites have applied
conical solid cylindrical mirror substrates.
For the next generation of planned X-ray satellites (XEUS,
CONSTELLATION-X) considerably larger telescopes are intended. Due
to cost considerations such telescopes cannot be manufactured
anymore as solid cylinders, but shall be composed of segmented
mirror segments. Two different replicating methods for the mirror
shells of these novel X-ray telescopes are currently intended to
proceed from the single production of current X-ray mirrors to a
line production and mass manufacturing for the planned satellites.
Both methods utilize precision formed bodies of the glass ceramic
ZERODUR.RTM. (glass ceramic), known as mandrels. The precision of
the mandrel is transferred to the mirror shell segments by means of
a nickel galvanic method (XEUS) or by means of duplicating
utilizing an epoxy synthetic as an intermediate layer
(CONSTELLATION-X). Both processes are performed at low temperatures
below 100.degree. C.
For the mentioned novel replicating processes for the manufacture
of segmentized X-ray mirrors preformed glass substrates are
necessary. This holds true for the planned project of NASA
CONSTELLATION-X, as well as for the planned satellite of ESA XEUS,
however, with respect to the latter only as an alternative
technology for the favored galvanic method.
The mentioned mandrels of ZERODUR.RTM. (glass ceramic), are not
suitable as molds for the manufacture of preformed glass
substrates, since the operating range of ZERODUR.RTM. (glass
ceramic), is 600.degree. C. at the most, while already starting
from 130.degree. C. particular restrictions are present.
However, for a cost-effective molding of mirror substrates
temperatures are necessary which, in part, are considerably above
600.degree. C.
On the other hand, sintered ceramic molds cannot be manufactured in
particular at large dimensions with the necessary form precision
and precision (e.g. freeness from pores).
However, the manufacture of quartz molds is very expensive and
restricted to smaller dimensions.
From EP 1,170,264 A1 it is basically known that glasses of the
system Li.sub.2O--Al.sub.2O.sub.3--SiO.sub.2 can be transformed
into glass ceramics (LAS glass ceramics) with high quartz mixed
crystals and/or keatite mixed crystals as predominant crystal
phases. These glass ceramics are prepared in various steps. After
melting and hot forming usually the material is cooled below the
glass transition temperature. Thereafter the base glass is
transformed into a glass ceramic article by controlled
crystallization. This ceramization is performed by an annealing
process having several steps in which in the beginning nuclei are
formed by nucleation at a temperature between 600 and 800.degree.
C., usually from TiO.sub.2 or ZrO.sub.2/TiO.sub.2 mixed crystals.
Also SnO.sub.2 may take part in the nucleation. During a subsequent
raise of temperature high quartz mixed crystals grow on these
nuclei at a crystallization temperature of about 750 to 900.degree.
C. Herein the volume fraction between the crystalline high quartz
mixed crystal phase and the glassy phase can be controlled in such
a way that a coefficient of expansion of about 0 is reached. To
this end normally a fraction of about 80% high quartz mixed
crystals to about 20% residual glass is desired.
According to EP 1,170,264 A1 a short-time temperature increase up
to 1100.degree. C. or more is performed, whereby the glass ceramic
is transformed into a ceramic having predominantly a keatite mixed
crystal phase in the core and having a high quartz mixed crystal
phase close to the surface.
However, the application of this glass ceramic disclosed herein is
limited to cooking surfaces, cooking utensils, fire-proof glass
etc.
SUMMARY OF THE INVENTION
It is a first object of the invention to disclose a method for the
forming of glass or glass ceramics that is particularly suited for
the manufacture of a mirror substrate, in particular for an X-ray
mirror.
It is a second object of the invention to disclose a method for the
forming of glass or glass ceramics, that allows the production of
formed bodies in a cost-effective and efficient way.
It is a third object of the invention to disclose a method for the
forming of glass or glass ceramics, that allows the production of
formed bodies at a high precision which may be in the order of a
surface form tolerance of 30 .mu.m or even lower.
It is a forth object of the invention to disclose a mold and a
process for making such a mold that can be utilized in the forming
of glass or glass ceramics.
It is a fifth object of the invention to disclose a mold suitable
for a sagging process of glass or glass ceramics at temperatures in
excess of 600 .degree. C.
These and other objects of the invention are reached with respect
to the mold for hot forming of glass or glass ceramics by preparing
the mold as a glass ceramic mold from a base glass by casting, the
base glass being heat-treated for transforming into a keatite glass
ceramic comprising predominantly a keatite mixed crystal phase.
With respect to the method for the forming of glass or glass
ceramics this object of the invention is solved by manufacturing a
formed body by sagging, under the action of gravity force, at a
temperature above the transition temperature of the blank, onto a
keatite glass ceramic form consisting of a lithium-aluminosilicate
glass ceramics comprising predominantly keatite mixed crystals.
The object of the invention is solved completely in this way.
The glass ceramic form of predominantly keatite phase manufactured
according to the invention exhibits a high temperature resistance
reaching up to approximately 1000.degree. C., wherein even higher
temperatures are possible for a short time. Thereby the mold
according to the invention can be utilized for the molding of,
e.g., a borosilicate glass in the sagging process at a temperature
that is considerably higher than the transition temperature. In
this way a high precision replicating of the mold consisting of
keatite glass ceramic can be reached. This is not possible with
prior art molds of the glass ceramic ZERODUR.RTM. (glass ceramic)
which comprises as its predominant crystal phase the high quartz
mixed crystal. By contrast to the common sintered ceramics, the
keatite glass ceramic is almost pore free and can thus be easily
polished. Pores at the mold surface would possibly produce
detrimental defects on the glass substrate while sagging. This is
avoided by the method according to the invention.
The coefficient of thermal expansion of keatite glass ceramic which
between 20.degree. C. and 700.degree. C. is in the range of about 1
to 3.times.10.sup.-6/K which is particularly suitable for the
forming of borosilicate glasses, since their coefficient of thermal
expansion is in the range of 3 to 4.times.10.sup.-6/K. By contrast,
the lower coefficient of thermal expansion of the glass ceramic
ZERODUR.RTM. (glass ceramic) which is about 0.2.times.10.sup.-6/K
between 20.degree. C. and 700.degree. C., is much more unfavorable
due to the bad matching with the coefficient of thermal expansion
of borosilicate glasses.
The novel manufacturing process for the hot forming of glass or
glass ceramics by utilizing a keatite glass ceramic mold leads to a
considerably improved precision during the manufacture of the
formed bodies, while the manufacturing process is simplified and at
the same time a molding by means of a complicated nickel galvanic
process is avoided.
It is even possible to cool the formed body in its transparent
state to room temperature in the beginning to allow an inspection
of the formed body with respect to its quality (freeness of pores,
inclusions, inhomogenities, striae etc.). At the request of a
particularly good homogeneity even an inspection with an
interferometer is possible.
The transformation into a keatite glass ceramic can be reached in a
subsequent heat treating step.
Since the keatite glass ceramic form is prepared from casted blank
glass blocks, also large and thick molds can be prepared. Thereby
the thickness of the mold also enhances the stiffness thereof and
guarantees a better form precision. The potential alternative
material quartz does not offer this possibility or only at
considerably higher cost of manufacture.
Highly stable keatite glass ceramic molds can be reached even at
large dimensions. Thus for instance diameters of 8 meters at a
thickness of 20 centimeters or more can be reached.
The keatite glass ceramic offers a good long time stability and a
high resistance against chemical environmental influences. A
hysteresis effect known in the glass ceramic ZERODUR.RTM. (glass
ceramic) does not occur in the keatite glass ceramic.
A glass ceramic mold according to the invention can be prepared
with the following steps:
casting a lithium-aluminosilicate base glass into a mold;
annealing for nucleation at a nucleation temperature of about 600
to 900.degree. C.;
annealing for the formation of a keatite glass ceramic at a keatite
formation temperature of about 800 to 1300.degree. C., until the
crystalline phase is transformed predominantly into keatite mixed
crystals;
cooling the glass ceramic mold thus formed to room temperature.
Herein basically it is possible, after annealing at nucleation
temperature, initially to anneal at a higher crystallization
temperature and to subsequently further increase the temperature,
to transform the initially formed high quartz mixed crystals almost
completely into keatite mixed crystals.
After casting of the base glass or after annealing for nucleation
or crystallization the body thus formed can be inspected in its
transparent state initially for its inner quality (bubbles,
inclusions, inhomogenities, striae etc.), before the transition
into an opaque body is performed by subsequent annealing.
Initially, by annealing at nucleation temperature and a subsequent
annealing at crystallization temperature a glass ceramic may be
prepared which contains predominantly high quartz as crystal phase.
Thus e.g. from the base glass for the preparation of a ZERODUR.RTM.
(glass ceramic) glass ceramic initially the ZERODUR.RTM. (glass
ceramic) glass ceramic with predominant high quartz mixed crystal
phase can be prepared, having a coefficient of expansion of
0.+-.0.15.times.10.sup.-6/K. By a subsequent heating and annealing
to the higher keatite formation temperature the high quartz mixed
crystals initially formed can be transformed almost completely into
keatite mixed crystals.
Alternatively, also it may be operated without an intermediate
cooling step, or after annealing for nucleation at a lower
temperature in the region of about 650 to 850.degree. C.
immediately it can be heated to the higher temperature necessary
for a keatite formation (in the region of about 800 to 1300.degree.
C.).
Also a three-step process cycle is possible by annealing initially
at nucleation temperature in the region of about 650 to 850.degree.
C., with a subsequent annealing at crystallization temperature in
the region of about 750 to 900.degree. C. (for forming high quartz
crystal phase), followed by an annealing at keatite formation
temperature in the region between about 850 and 1300.degree. C. for
effecting transformation of the high quartz mixed crystals into
keatite mixed crystals.
Annealing for keatite formation is preferably performed at a least
900.degree. C., preferably at at least 1000.degree. C. for at least
one hour, in particular for at least two hours, particularly
preferred for a time span of roughly four hours.
At even higher temperature the holding time may be shortened
correspondingly.
According to the method according to the invention annealing for
keatite formation is preferably performed at such a temperature and
for such a time that the crystalline fraction is largely
transformed into keatite. Preferably, herein at least 80 vol.-%, in
particular about 85 vol.-%, and particularly preferred at least
about 90 vol.-% of the crystalline fraction of the material is
transformed into keatite mixed crystals.
At the most then a glassy residual phase may exist which may be
enriched with high quartz mixed crystals and the insoluble
ingredients, such as Na.sub.2O and alkaline earth oxides such as
CaO, SrO, BaO. However, preferably any possible residual glassy
phase exists only in the form of inclusions which are dispersed
within a microstructure predominantly consisting of keatite mixed
crystals.
By such a structure the form stability and temperature stability
necessary for the hot forming of glasses can be guaranteed in the
region of 550.degree. C. up to about 1000.degree. C. If a larger
fraction of high quartz mixed crystal phase or glassy phase would
be present in the total body, then the form stability and
temperature stability at temperatures above 600.degree. C. or at
even higher temperatures could possibly be impaired.
According to a preferred development of the invention the glass
ceramic mold after casting of the base glass and/or after annealing
is mechanically finished, in particular ground, polished or
lapped.
In this way the necessary surface characteristics and form
precision can be reached by mechanical treatment (preferably with
CNC controlled machines) by operations known in the art of glass
processing. Even before the transformation into an opaque ceramic
occurs, a quality inspection can be performed in a particularly
simple way while still being in the transparent state.
According to another advantageous development of the invention the
sagging of the blank onto the glass ceramic mold is assisted by
applying a vacuum or an excess pressure.
In this way a particularly good form precision of <10 .mu.m can
be obtained.
In a suitable development of the invention the blank is
mechanically processed on both sides, preferably polished, before
sagging onto the glass ceramic mold.
Thereby a particularly high quality and precision of the formed
body thus prepared can be reached.
According to another advantageous development of the invention a
blank of a borosilicate glass, particularly BOROFLOAT.RTM.
(borosilicate glass, Schott Glas) glass, DURAN.RTM. (borosilicate
glass, Schott Glas) glass or PYREX.RTM. (borosilicate glass,
Corning) glass is sagged onto the glass ceramic mold at a
temperature between about 550.degree. C. and 850.degree. C.
In this way the advantageous characteristics of borosilicate glass
can be utilized for the manufacture of X-ray mirror substrates,
since borosilicate glass exhibits a very low tendency to
crystallization and can be processed very advantageously in the
given temperature range. Herein the temperature is preferably
considerably above the transition temperature of the particular
glass, whereby a high form precision and surface quality is reached
during replication even at a short holding time.
According to an advantageous development of the invention the
formed body is cooled after forming with a small cooling rate of
preferably at the most 1 K/min, in particular 0.5 K/min, more
particular 0.25 K/min, mostly preferred at about 0.1 K/min.
In this way a particularly stress-free formed body having a high
form precision is obtained.
For the manufacture of particularly high-quality formed bodies the
forming of the blank is preferably performed under clean-room
conditions.
In another advantageous development of the invention the contact
surface of the blank with the glass ceramic mold is utilized as the
back surface of a mirror to be prepared there from.
In this way any possible surface defects, which may emerge during
manufacture of the mirror substrate, are arranged on the back
surface of the mirror, thus being at a less critical region.
According to another advantageous development of the invention the
glass ceramic mold is designed concavely when the thermal
coefficient of expansion of the mold is smaller than that of the
blank.
This is usually the case with commonly utilized borosilicate
glasses.
Herein a concave mold having a cone profile, a hyperbolic profile,
a parabolic profile (Wolters profile) is utilized, in case an
application as an X-ray mirror substrate is desired.
However, if the coefficient of thermal expansion of the glass
ceramic mold is larger than that of the blank, then the mold is
preferably designed convexly.
After forming the formed body may be mechanically finished,
preferably polished, to obtain a surface quality as good as
possible.
In case a utilization as a mirror substrate is intended, then a
reflective coating may be applied to the formed body.
As a base glass for the manufacture of a keatite glass ceramic mold
preferably a glass is utilized comprising the following components
(in weight percent):
TABLE-US-00001 SiO.sub.2: 35 75 Al.sub.2O.sub.3: 17 32 Li.sub.2O: 2
8 B.sub.2O.sub.3: 0 5 P.sub.2O.sub.5: 0 17 SnO.sub.2 + ZrO.sub.2 +
TiO.sub.2: 0.1 7 Na.sub.2O + K.sub.2O + Cs.sub.2O: 0 6 CaO + MgO +
SrO + BaO + ZnO: 0 8 refining agents such 0 3 as Sb.sub.2O.sub.3,
As.sub.2O.sub.3, SnO.sub.2, CeO.sub.2, sulfate or chloride
compounds: coloring oxides such 0 10. as V.sub.2O.sub.5,
Cr.sub.2O.sub.3, MnO, Fe.sub.2O.sub.3, CoO, NiO and other
oxides:
Herein particularly preferred a base glass is utilized comprising
the following components (in weight percent):
TABLE-US-00002 SiO.sub.2: 55 70 Al.sub.2O.sub.3: 19 25 Li.sub.2O:
2.5 4.5 B.sub.2O.sub.3: 0 1 P.sub.2O.sub.5: 0 8 SnO.sub.2 +
ZrO.sub.2 + TiO.sub.2: 0.5 5 Na.sub.2O + K.sub.2O + Cs.sub.2O: 0.1
3 CaO + MgO + SrO + BaO + ZnO: 0 5 refining agents such 0 2 as
Sb.sub.2O.sub.3, As.sub.2O.sub.3, SnO.sub.2, CeO.sub.2, sulfate or
chloride compounds: coloring oxides such 0 2. as V.sub.2O.sub.5,
Cr.sub.2O.sub.3, MnO, Fe.sub.2O.sub.3, CoO, NiO and other
oxides:
With such a lithium alumino silicate base glass the desired
predominant forming of a keatite phase in the keatite glass ceramic
body can be obtained. Silicon oxide, aluminum oxide and lithium
oxide are all necessary in the given range to effect some
crystallization and a low thermal expansion. These components are
components of the keatite mixed crystals. The Al.sub.2O.sub.3
contents should preferably not be higher than 19.8 weight percent,
since otherwise high SiO.sub.2 contents of residual high quartz
mixed crystals could be promoted. By contrast, too high a
Al.sub.2O.sub.3 content could lead to undesired deglassing of
mullite. Preferably, boron oxide is not added at all or only in
small amounts, since higher boron oxide contents are detrimental
for the crystallization. P.sub.2O.sub.5 may be added as a further
component. Mandatory is the addition of ZrO.sub.2 or TiO.sub.2 as
nucleation initiators. Alternatively or in addition also SnO.sub.2
may be added. The addition of the alkalis Na.sub.2O, K.sub.2O,
Cs.sub.2O as well the alkaline earths CaO, SrO, BaO improves the
meltability and the deglassing characteristics of the glass during
manufacture. MgO and ZnO act in a similar way. The glass ceramic
may be prepared while adding common refining agents, such as e.g.
As.sub.2O.sub.3, Sb.sub.2O.sub.3, SnO.sub.2, CeO.sub.2, NO.sub.2,
CeO.sub.2, sulfate or chloride compounds, such as NaCl. Also
coloring oxides, such as V.sub.2O.sub.5, Cr.sub.2O.sub.3, MnO,
Fe.sub.2O.sub.3, CoO, NiO or other oxides may be present in the
given ranges.
Preferably, a composition may be utilized which corresponds to the
known composition of ZERODUR.RTM. (glass ceramic) or ZERODUR-M
(glass ceramic) sold by the applicant. In addition, also other
similar glass ceramics may be utilized as a base glass, such as
CERAN.RTM. (glass ceramic, Schott Glas), ROBAX.RTM. (glass ceramic,
Schott Glas), CLEARCERAM.RTM. (glass ceramic, Ohara, Inc.),
NEOCERAM (glass ceramic, NeoCeram SA), ASTROSITALL (glass ceramic,
Lytkarino Optical Glass Factory).
As mentioned above, the glass ceramic mold of keatite ceramic
according to the invention is particularly suitable for the
manufacture of formed bodies from glass for the preparation of
X-ray mirrors, in particular for telescope applications, by
applying a sagging process.
X-ray mirror substrates made of glass offer particular advantages
over other substrate materials, due to their high self stiffness,
low expansion, low density and thereby higher thickness at equal
mass, good polish ability, as well as a good availability also at
large dimensions and a simple processing by known processing
operations. In addition, also an inspection for inner stresses can
be effected on transparent formed bodies after the replication
process. Inner stresses within the mirror substrate could lead to a
deformation, to a bending or warping, so that the necessary precise
shape of the mirror segments could not be reached anymore. A
transparency of the processed formed body also allows for an
inspection of inner glass qualities.
It will be understood that the above-mentioned and following
features of the invention are not limited to the given
combinations, but are applicable in other combinations or taken
alone without departing from the scope of the present
invention.
Further features and advantages of the invention will become
apparent from the following description of preferred embodiments
taken in conjunction with the drawings. In the drawings:
FIG. 1 is a schematic representation of a device for vacuum sagging
utilizing a glass ceramic mold according to the invention, shown in
the initial state;
FIG. 2 shows the device according to FIG. 1 in its final state in
which the blank has set against the surface of the mold.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to the invention a mold for the hot forming of glass or
glass ceramics is prepared as a glass ceramic mold from a base
glass by casting, this being transformed into a keatite glass
ceramic of predominant keatite phase by means of annealing. To this
end a base glass can be utilized preferably comprising the
following components (in weight percent):
TABLE-US-00003 SiO.sub.2: 55 70 Al.sub.2O.sub.3: 19 25 Li.sub.2O:
2.5 4.5 B.sub.2O.sub.3: 0 1 P.sub.2O.sub.5: 0 8 SnO.sub.2 +
ZrO.sub.2 + TiO.sub.2: 0.5 5 Na.sub.2O + K.sub.2O + Cs.sub.2O: 0.1
3 CaO + MgO + SrO + BaO + ZnO: 0 5 refining agents such 0 2 as
Sb.sub.2O.sub.3, As.sub.2O.sub.3, SnO.sub.2, CeO.sub.2, sulfate or
chloride compounds: coloring oxides such 0 2. as V.sub.2O.sub.5,
Cr.sub.2O.sub.3, MnO, Fe.sub.2O.sub.3, CoO, NiO and other
oxides:
Initially, the lithium-aluminosilicate base glass is molten,
possibly while adding common refining agents, and is cast into a
mold.
Thereafter for example firstly after cooling to room temperature
the surface characteristics and the inner characteristics may be
inspected for possible inclusions, such as pores, bubbles, striae
etc., and possibly a mechanical treatment by grinding, polishing or
lapping may be performed.
Subsequently firstly annealing for nucleation is performed at
nucleation temperature in the region of about 600 to 900.degree.
C., preferably in the region of about 600 to 800.degree. C. Herein
starting from the nucleating agents TiO.sub.2, ZrO.sub.2 or
SnO.sub.2 nuclei are formed in large numbers. By contrast, also
before this a cooling to room temperature can be performed, this
also leading to additional nuclei. Only during a subsequent
annealing at a higher temperature a crystallization occurs, i.e. a
growing of the nuclei, to form the crystalline phase. Also this can
initially be performed in a region suitable for the forming of a
high quartz crystal phase, namely in a temperature region of about
700 to 900.degree. C. If subsequent cooling to room temperature is
performed, this yields a glass ceramic having as crystal phase
predominantly high quartz mixed crystals. If this glass ceramic is
subsequently heated to a higher temperature, necessary for keatite
formation, namely to a temperature range between about 800 and
1300.degree. C., preferably to a temperature of at least
1000.degree. C., then the high quartz mixed crystals formed before
largely transform to keatite mixed crystals while growing
simultaneously. The annealing at keatite formation temperature
herein preferably is performed at a sufficiently high temperature
and for a sufficiently long time to ensure a largely complete
transformation of the crystal phase into a keatite mixed crystal
phase. E.g., this may be ensured by annealing at about 1000.degree.
C. for a time of at least one hour, e.g. for four hours.
Subsequently cooling to room temperature is effected.
Alternatively also the keatite glass ceramic may be prepared
without any immediate cooling, after annealing at nucleation
temperature, by immediately heating to the higher temperature
necessary for keatite formation at which a sufficient holding time
is spent. However, since an opaque body is formed hereby, a
subsequent (optical) inspection for the inner quality of the
keatite glass ceramic mold formed thereby is complicated.
The resulting coefficient of expansion of the keatite glass ceramic
is in the range of about 1.times.10.sup.-6/K to
3.times.10.sup.-6/K, however, preferably of about
2.0.times.10.sup.-6/K, between 20.degree. C. and 700.degree. C.
For the preparation of mirror substrates, in particular for X-ray
mirrors for telescope applications the keatite glass ceramic mold
resembles the inverse form of the necessary mirror substrate,
wherein differences in the coefficients of expansion between mold
and glass substrate must be taken into account and in and
correspondingly included in the shape of the mold. Preferably,
herein the difference between the two coefficients of expansion is
kept as small as possible, and the coefficient of expansion of the
keatite glass ceramic mold is matched during manufacture
correspondingly. If the coefficient of expansion of the mold is
larger than that of the glass, preferably, a convex form is
utilized. However, if the coefficient of expansion of the mold is
smaller than that of the glass, such as when utilizing a
borosilicate glass, then a concave shape may be utilized. For the
forming of high precision mirror substrates preferably the later
mirror backside is in contact with the mold to avoid surface
defects due to contacts on the mirror side.
Preferred materials for the mirror substrates are borosilicate
glasses (BOROFLOAT.RTM. (borosilicate glass), DURAN.RTM.
(borosilicate glass), PYREX.RTM. (borosilicate glass)) having a low
coefficient of expansion of 3 to 4.times.10.sup.31 6/K. Preferably,
the glass substrate is initially polished on both sides to reach a
small variation in thickness of the glass and a flat surface.
Usually the glass substrate is applied onto the mold as a flat
plate, and subsequently both are heated together according to a
heating program up to a temperature above the transition
temperature of the substrate glass (T>T.sub.g). For some
particular borosilicate glasses (BOROFLOAT.RTM., (borosilicate
glass)) the necessary sagging temperature herein is between
560.degree. C. and 760.degree. C. Preferably, the cooling to room
temperature is performed at small cooling rates of about 0.5 K/min
to avoid the formation of inner stresses within the glass
substrate. A correspondingly good temperature homogeneity within
the furnace must be ensured. Also to obtain the surface quality
necessary for X-ray mirror substrates, the contact surfaces of the
mold and the substrate should be kept particularly clean.
Therefore, the sagging process is preferably performed under clean
room conditions to avoid any dust particles. By sagging under
gravity force a precision of >30 .mu.m can be reached. By the
assistance of a vacuum or possibly by an excess pressure a form
precision of >10 .mu.m or even lower is possible. During sagging
it should be taken into account that the sagging temperature must
be matched to the sagging geometry. At other thicknesses and other
dimensions the sagging temperature must be adjusted
accordingly.
EXAMPLE
A base glass comprising the following components (in weight
percent) was molten:
TABLE-US-00004 SiO.sub.2 55.50 Al.sub.2O.sub.3 25.30 P.sub.2O.sub.5
7.90 Li.sub.2O 3.70 Na.sub.2O 0.50 MgO 1.00 ZnO 1.40 TiO.sub.2 2.30
ZrO.sub.2 1.90 As.sub.2O.sub.3 0.50.
This corresponds to a possible composition of the glass ceramic
sold by the applicant under the trademark ZERODUR.RTM. (glass
ceramic). The base glass manufactured in this way after refining
was cast into a blank glass block and thereafter ceramized by
controlled crystallization, while utilizing a temperature program.
To this end initially heating up to 730.degree. C. was performed at
0.1 K/min, 730.degree. C. were maintained for a time period of 24
hours, subsequently heating up to 850.degree. C. was performed at
0.1 K/min, this followed by a further holding at 850.degree. C. for
48 hours, this followed by a slow cooling to room temperature at
0.1 K/min.
Depending on the size of the glass ceramic block this temperature
profile must be adjusted accordingly to yield a high precision
crack-free glass ceramic having a high quartz mixed crystal phase
in the desired range.
The crystallization is an exothermal process during which the
material shrinks. For the manufacture of large glass ceramic blocks
a homogenous temperature distribution with a local temperature
deviation of 2 K at the most must be obtained across the complete
glass ceramic block. Thus the ceramization process of large blocks
of several meters diameter may take up to nine months.
From a thus manufactured glass ceramic block of predominant high
quartz crystal phase a suitable blank part was cut out,
mechanically processed at its surface and inspected for its
quality.
Thereafter heating up to 1000.degree. C. was performed at 1 K/min,
followed by a holding at this temperature for a time of four hours,
before a controlled cooling to room temperature was performed at 1
K/min.
The keatite glass ceramic manufactured thereby was completely
opaque and resembled only a small glassy residual fraction, while
the crystalline phase had almost completely transformed into
keatite mixed crystal phase. The mold manufactured thereby was
mechanically processed and finally lapped to yield the necessary
shape and surface characteristics.
Within a furnace which is shown in FIGS. 1 and 2 and designated in
total with numeral 10, a borosilicate glass blank was replicated
from this mold.
The furnace 10 is a furnace known in the art allowing for a
homogenous temperature distribution and a selective heating and
cooling at very slow heating rates and cooling rates,
respectively.
In such a furnace 10 a borosilicate blank 14 of BOROFLOAT.RTM.
(borosilicate glass) was sagged at about 650.degree. C. onto the
keatite glass ceramic form 12 prepared before, without the
application of a vacuum.
With such a sagging process the form tolerance of the shaped body
14'formed thereby can reach >30 .mu.m when utilizing
BOROFLOAT.RTM. (borosilicate glass) glass plates polished on both
sides.
To effect an even better form tolerance of <10 .mu.m, a vacuum
sagging is preferred, as indicated in FIGS. 1 and 2. Herein the
keatite glass ceramic form 12 is preferably prepared already with
one or more vacuum connections 16 to which later, as shown in FIG.
2, during hot forming of the glass substrate 14' a vacuum is
applied. Herein the blank 14 sets against the surface 18 of the
keatite glass ceramic form 12 with a good form precision of <10
.mu.m.
* * * * *